Factors Affecting Variation in the Egg and Duckling Components of Wood Ducks

We collected 3 eggs from each of 35 female Wood Ducks (Aix sponsa). Fresh-egg mass averaged 44.2 g and consisted of 53.1% albumen, 36.4% yolk, and 9.6% shell. Albumen and yolk contained 86.2% and 44.9% water, respectively. Lipids comprised 65.1% of the dry yolk. All egg components except dry albumen increased in direct proportion to fresh-egg mass. Variation among females explained 52-80% of the total variation in mass and composition of eggs. Body mass of hens during early incubation was correlated with estimates of pre-egg-laying lipid reserves ($r_{{\rm s}}=0.66$); therefore, body mass was a good measure of female quality. Body mass of females was independent of age and structural size, and was positively related to mean egg mass, egg composition, energy content of eggs (kJ/g), and clutch mass, but not to clutch size or time of nesting. Female body mass explained more variation in albumen components than in yolk or shell components. These results support Drobney's (1980) hypothesis that prebreeding condition of female Wood Ducks is important because it allows hens to accumulate exogenous protein for egg synthesis. The data do not support predictions based on optimal egg size theory. The body mass of ducklings averaged 23.7 g (n = 43); there were no intersexual differences in mass or composition. Ducklings contained 65.9% water and 32.5% lipids (dry mass). Components of ducklings increased in direct proportion to fresh-egg mass, but egg mass was a poor predictor of duckling lipid content.